test: mark test-fs-read-buffer-to-string-fail as flaky

PR-URL: https://github.com/nodejs/node/pull/14495
Fixes: https://github.com/nodejs/node/issues/14430
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/parallel.status

@@ -5,6 +5,7 @@ prefix parallel
 # sample-test                        : PASS,FLAKY
 
 [true] # This section applies to all platforms
+test-fs-read-buffer-tostring-fail      : PASS,FLAKY
 
 [$system==win32]